1. Technical Field
The present invention relates to a color conversion device, a color conversion table, and a color conversion method, and particularly to a color conversion device which converts input color data in a first color gamut into color data in a second color gamut and then outputs the color data.
2. Background Art
In order to achieve color reproduction between different devices and different media, it is necessary to correct a difference in color gamut between input and output devices. A technique of correcting the difference in color gamut between the input and output devices is referred to as either color gamut conversion or color gamut compression and expansion.
For example, as display output devices which perform image display, there are CRTs (Cathode Ray Tubes), PDPs (Plasma Display Panels), LCDs (Liquid Crystal Displays), and panels such as projector screens. Each of these has a different color gamut as a reproducible color gamut.
Since each device has a different reproducible color gamut, it is necessary, when, for instance, causing a display output device to display an image signal, to correct the image signal to a color gamut corresponding to the display output device.
In order to correct the difference in color gamut, it is common to convert the signal values from individual device or medium, represented by R, G, and B, into values of device-independent color system such XYZ, L*a*b*, and L*u*v* color spaces, for instance, CIE (Commission Internationale de l'Eclairage: International Commission on Illumination), and to perform compression and expansion on the values of the device-independent color system.
Meanwhile, color gamut compression methods can be mainly classified into compression and clipping.
Compression is a method for compressing all colors. In other words, in compression, when a first color gamut is compressed into a second color gamut, color conversion is performed by compressing all colors within the first color gamut so that all the colors are included in the second color gamut (e.g., refer to Patent References 1 to 3).
It is to be noted that the compression includes (i) a method in which colors within and out of a color gamut are linearly compressed and (ii) a non-linear compression method in which the compression rate is increased as the saturation of a color increases. In addition, there are disclosed methods for changing a compression ratio according to hue, saturation, or value, dividing a color gamut into regions in which colors are maintained and regions in which no color is maintained, and compressing saturation in each of the divided regions with each different Linear transformation (e.g., refer to Patent References 4 to 7).
On the other hand, clipping is a method for (i) faithfully reproducing (not compressing), among colors in the first color gamut, colors included in the second color gamut, that is, colors that can be faithfully reproduced by a reproducing device and (ii) compressing only non-reproducible colors, that is, colors that are not included in a color gamut of the reproducing device (e.g., refer to Patent Reference 8).
It is to be noted that, for clipping, there are disclosed a method for (i) compressing a color that cannot be reproduced in the color gamut of the reproducing device such that a color difference between the color before compression and the color after compression is minimum and (ii) reproducing the color before compression, and a method for performing compression such that colors of a neighborhood in value or saturation are selected (e.g., Patent References 9 and 10).
Furthermore, for color gamut expansion, there are disclosed a method in which, for instance, when an output color reproduction range with respect to an input image is broader than an input color reproduction range in terms of color system values, extension mapping is performed on the color system values of the input image so that the input color reproduction range is expanded according to a ratio between the input and output color reproduction ranges, after which the color system values are outputted (e.g., refer to Patent Reference 11), and a method of (i) transforming color coordinates by expanding color attributes of value (L*) and chroma (C*) using a linear extension equation and (ii) performing control so that the transformed color coordinates remain within a color gamut of a device (e.g., refer to Patent Reference 12).    Patent Reference 1: Japanese Unexamined Patent Application Publication No. 60-105376.    Patent Reference 2: Japanese Unexamined Patent Application Publication No. 61-288690.    Patent Reference 3: Japanese Unexamined Patent Application Publication No. 63-254889.    Patent Reference 4: Japanese Unexamined Patent Application Publication No. 06-225130.    Patent Reference 5: Japanese Unexamined Patent Application Publication No. 61-288662.    Patent Reference 6: Japanese Unexamined Patent Application Publication No. 07-220067.    Patent Reference 7: Japanese Unexamined Patent Application Publication No. 2003-283846.    Patent Reference 8: Japanese Unexamined Patent Application Publication No. 63-195777.    Patent Reference 9: Japanese Unexamined Patent Application Publication No. 04-040072.    Patent Reference 10: Japanese Unexamined Patent Application Publication No. 2000-278546.    Patent Reference 11: Japanese Unexamined Patent Application Publication No. 03-158075.    Patent Reference 12: Japanese Unexamined Patent Application Publication No. 2003-153027.